When data packets are transmitted through networks, both network-layer addresses, e.g. Internet IP addresses, and link-layer addresses are used. The network layer is responsible for routing data packets from one host to another. The network layer routes a data packet through a series of packet switches, called routers in the Internet, between the source and destination. To move a packet from one node, e.g. a host or a packet switch, to the next node in the route, the network layer must rely on the services of the link layer. In particular, at each node, the network-layer protocol passes the data packet to the link-layer, which delivers the data packet to the next node along the route. At this next node, the link-layer passes the data packet to the network layer. The services provided at the link-layer depend on the specific link-layer protocol employed over the link.
Due to the fact that there are both network-layer addresses, for example Internet IP addresses, and link-layer addresses, there is a need to translate between them. For the Internet with IPv4, this is achieved by the address resolution protocol (ARP) as defined in the Internet Engineering Task Force (IETF) specification RFC826. Every Internet host and router has an ARP module. For the Internet with IPv6, this is achieved by the address resolution procedure as defined in the Internet Engineering Task Force (IETF) specification RFC2461.
In connectionless access link networks that support broadcasting at link level, like Wireless Local Area Network (WLAN) or Ethernet, an access router (AR) can send link level frames containing IP data packets in broadcast and every host attached to the link can pick up the frame. The IP packet contained within the frame can itself be addressed to a single host via or multiple ones via an IP multicast address.
In cellular based systems supporting mobile nodes, there is an entity between the AR and the final host, e.g. mobile node. This entity is the access node or base station and is called hereinafter a cellular access point (CAP). The CAP provides link-level connectivity to the mobile node. This cellular access is connection-oriented and the CAP provides all features used to support these connection-oriented principles, e.g. quality of service (QoS) and reservation of resources. By connection-oriented it is meant that before data packets can be exchanged between a mobile node and the CAP, it is necessary to establish an association between both of them via radio signalling. This signalling is used to set up a radio bearer, which provides the radio resources to transmit the data packets. Because of this connection-oriented nature, multicasting a data packet to many mobile nodes requires sending one copy on each radio bearer, e.g. on each association between the CAP and the mobile node.
FIG. 4 shows a schematic diagram indicating an address resolution scheme provided at an AR 20 for converting an incoming IP data packet 60 into a link-layer frame 70 forwarded to the mobile node. At the AR 20, a routing table 80 is provided the entries of which provide a link between the network-layer destination address (DA) of the incoming IP data packet 60 and a respective interface (I) for routing the data packet to the mobile node. Furthermore, the AR 20 provides a neighbour cache 90 for each interface, in which respective link-layer addresses (MN_LLA) are allocated to the supported destination addresses.
When the IP data packet 60 which comprises a header portion 61 and a payload portion 62 arrives at the AR 20, the AR 20 derives the respective routing interface from the routing table 80 and uses the neighbour cache of the derived interface to obtain the corresponding link-layer address (“MN_LLA”) of the mobile node. Then, the AR 20 encapsulates the received IP data packet 60 into the payload portion 72 of a link-layer frame 70 having a header portion 71, and adds the derived link-layer address “MN_LLA” of the mobile node to the header portion 71 of the link-layer frame.
In the example shown in FIG. 4, the incoming IP data packet 60 comprises a destination address “A:B:C:D:E:F:G:H”, i.e. an IPv6 address, based on which the interface and the link-layer address are obtained at the AR 20. Accordingly, the link-layer address “MN_LLA” is added to the header portion 71 of the link-layer frame 70 and the original network-layer destination address “A:B:C:D:E:F:G:H” is conveyed in the payload portion 72 of the link-layer frame 70, i.e. in the header portion 61 of the encapsulated IP data packet 60. The AR 20 sends the encapsulated IP data packet 60 together with the link-layer frame 70 to the link-layer address “MN_LLA” of the desired mobile node. The link-layer frame 70 then arrives at one or several CAPs allocated to the same AR 20, and the CAPs check the link-layer address “MN_LLA”, while only the CAP supporting the addressed mobile node picks up the link-layer frame 70 with the IP data packet 60 and delivers it to the addressed mobile node.
When an IP data packet addressed to the IP address of a mobile node arrives at the AR 20 and the AR 20 does not know the respective link-layer address, this event provokes the address resolution procedure at the network layer which procedure needs to multicast a neighbour solicitation message addressed to a network-layer address for multicast purpose, i.e. a solicited-node multicast address' corresponding to the target IP address. Thus, the IP data packet to be sent is not the incoming IP data packet but the neighbour solicitation message used for learning the link-layer address of the mobile node. At link level, the link-layer frame is addressed to a well know link-layer address for multicast purposes, since the AR 20 is not aware of the link-layer address of the mobile node. This message will reach every mobile node but only those joining this network-layer address for multicast purpose will process the neighbour solicitation message. However due to the connection-oriented nature of the cellular-based system, there is no possibility to broadcast this kind of messages, i.e. messages with multicast purpose in general, to every mobile node. Thus, the IP data packets have to be forwarded to every mobile node one by one over the radio link. This leads to an increased load and wasted bandwidth of the radio link.